A study of the complex genetics of left-right (L-R) axis malformations has been undertaken with an emphasis on those genes that are associated with common phenotypes of L-R disorders, including situs inversus, heterotaxia and organ isomerism. L-R defects can result from either environmental or genetic causes and it is the aim of these investigations to determine the genes responsible for both normal and abnormal L-R axis formation through the study of patients with these disorders. Mutations in several such genes have been shown to be responsible for several familial and sporadic cases of heterotaxia: ZIC3, LEFTY A, ACVR2B, and CFC1. We have identified several additional genes important for L-R development including FAST1 and GDF1.